Potentiometer winder



H. HARRIS, JR POTENTIOMETER WINDER June 23, 1953 2 sheets-sheet 1 Filed June 9, 1944 lNvENTo ATTORNEY June 23, 1953 H. HARRIS, JR

POTENTIOMETER wINbER 2 sheets-sheet 2 Filed Jun'ele.' 1944 ATI'ORNE'Y INVENTOR www Patented June 23, 1953 POTENTIOMETER WINDER Herbert Harris, Jr., ,CedarhurstgN Y., assignor `to The Sperry Corporation, acorporat'ion of Dela- Ware Application June 9, 1944, Serial No.539,412 4Clalin1s. (Cl. 242-3) `My invention relates to methods 'and apparatus for :winding resistance cards for use with potentiometers, rheostats, and so forth,

An object of the invention is to provide-accu rate functional potentiometers including, for example, precisely linear potentiometers.

Another object is to provide apparatus for continuously measuring resistance and continuously comparing resistance of av'rheostat or potentiometer card with a standard or master card whilethe card is being wound.

Still :another object is to provide arrangements for automatically varying Vthe Winding pitch vof a potentiometer as it is being wound in order to produce an accurate predetermined functional relationship between :resistance and linear distance measured along the potentiometer.

Still another object is to provide arrangements for either varying the speed of rotation or varying the .lead as afpotentiometer -'is being Wound in order lto correct `for any :deviations from Va predetermined `functional resistance-distancc relationship.

Sti-ll another object of the invention isto pro@ vide `arrangements vwhereby potentiometers may be 'wound accurately veven though `the resistivity or the diameterl ofthe resistance wire employed may not be uniform and winding 4torms employed may deviate from la specified shape land size as by the use of tapered cards. v.furthermore, it is an object to provide potentometers having accurate resistance-values at each point and .not merely accurate resistance ratios between various points.

A further object is to provide a speed control system in `which the :speed setting may readily n be varied. 1

`Another object Aisto provide an improved electronic motor speed control system;

Another object is to provide almotor speed control system responsive :selectively to manual 'setting guided by a monitoringvoltage or auto-l matically responsive Vto `variation in la, quantity to be controlled. Y

Another object is to provide an automatic', speed control system in which the loadgon the automatic control produced by a persistent er# ror may be relieved by a manual control.

A further object is to provide combined manual and automatic pitch control.

ther and further objects,` features and advantages of the invention will becomeapparent as the description proceeds.

Functional potentiometers have been employed `in which a desired functional relationtentiometers for use where high precision is nec-V essary. Itis accordingly an object of my in; vention to'provide improved more accurate functional potentiometers, and to provide improved methods and apparatus for exactly winding each potentiometer to `specifications 'without leaving the attainment of `the desired functional relationshipto chance. It is, of course, immaterial whether the unit vwound is to be used as a rheostat, or `as a potentiometer in the strict literal sense. Howeven for .convenience I have used the expression potentiometer in the description and vclaims broadly tosignify any resistance unit which 'is to be produced Iwith'accurate resistances from ipoint lto point. k

'l In carrying out the :invention in its preferred form, a potentiometer winding machine is employed which maybe of conventional'design in so far as it employs conventional chucks for holding the `card or :other support on which resistance wire is to be wound together with a conventional lead screw arrangement for causing the wire to be rfed to successively different points along the Icard or form. on which it is being Wound. v

l However, l. also provide means for continu ously measuring the resistance of the wire al ready wound on the card, and I'provide a mas ter potentiometer or card with means for moving its brush 'so as'to produce a desired functional relationship between brush motion and resistance, For correcting any inaccuracies in the resistance of the potentiometer being wound, I provide means'for comparing this resistance with that of the master potentiometer, and I provide a motor speed adjusting `mechanism responsive to any unbalance for varying the ratio of lead speed to `Winding speed by varying either the speed of the lead screw or the speed at which the card holding chucks are driven,in order to vary'the pitch of the potentiometer :being wound,

Preferably, Ythe .potentiometer to be wound is so A designed that `the clearance between successive turns of wire, which may be uninsulated wire, is about twice the thickness of the wire so that there is ample tolerance for variation in pitch to correct for any variations in resistivity or diameter of the wire. The card, cylinder or other form upon which the wire is wound may either be or uniform width or diameter or have an irregular shape according to the resistance function desired, but the winding form may be of uniform width even where nonlinear function is desired since the apparatus automatically adjusts the pitch to produce a desired functional relationship.

A better understanding of my nivention will be afforded by the followingy detailed description considered in conjunction with the accompanying drawings and those features of the invention which are believed to be novel and patentable will be pointed out in the claims appended hereto.

In the drawings,

Fig. 1 is a perspective view, partially scheimatic, and with portions of the apparatus omitted for clarity, illustrating one embodiment of the invention; and

Fig. 2 is a circuit diagram of the apparatus of Fig. 1 with mechanical connections indicated schematically.

Like reference characters are utilized through the drawings to designate like parts.

The invention is by no means limited to the use of any particular type of winding -apparatus or to the specific electric circuit arrangements shown by way of illustration. The invention is not limited to winding apparatus in which wire is wound upon a rotatable winding form instead of by a wire feed revolving about a stationary winding form, such as employed in connection with toroidal windings, for example. Nevertheless, for simplifying the explanation I have shown in the drawings a conventional rotating-windingform type of winding machine modied and provided with additional attachments for the purpose of automatically controlling the resistance of the potentiometer being wound, thus illustrating one way'of carrying out my invention.

The apparatus illustrated comprises a winding machine represented generally by the reference numeral II, with a drive motor I2 and in addition thereto, an error correction motor I3 with electrical control apparatus I4. The latter is shown in Fig. 1 in its enclosing casing having electrical connections through cables, omitted for the sake of clarity, to the motors and potentiometers shown in the figure.

The winding machine II has conventional end plates I5 supporting a drive shaft I5, chuck shafts I1 and I8, a cross rod I9 and a lead screw 2I. Card holding chucks 22 and 23 yare mounted upon the shafts I1 and I8, respectively, and are mechanically connected to the drive shaft I6 through conventional gearing arrangements 24 and 25.

For feeding a bare resistance wire 25 and traversing it along a card or other winding form 21 composed of insulating material, a lead or wire feeding carriage 28 is provided which is movably Supported upon the cross rod I9 and the lead screw 2I. For causing the threads of the lead screw 2I to advance the wire feeding carriage 28, a Asuitable thead engaging mechanism 29 is provided, having a release lever 3U.

A pair of rollers 3I is provided for passing the wire 25 to' the card 21. For guiding the wire to the desired axial position with respect to the 4 card 21, a vertical post 32 and a guide pulley 33 are provided. It is to be understood that the incoming length of wire 34 is drawn from a supply spool (not shown).

To enable electrical measurement of resistance to be made, the chuck 22 is preferably so constructed that it is electrically insulated from the gearing 24 and from the supporting chuck shaft I1, and a slip ring 35 is provided having a terminal screw 36 to which the end of the wire on the card 21 may be connected mechanically and electrically. For making slidable electrical connection to the slip ring 35, a metallic brush 31 is provided which is insulated from a supporting bracket-38 and has a conductor 39 electrically connected thereto. For preventing the wire from being grounded to the winding apparatus, the rollers 3| are preferablycomposed of insulating material or are insulated from the supporting members, and the members 32 and 33 are likewise eitheir insulated or composed of insulating material. For making electrical contact to the portion of the wire passing the post 32, a spring contact 4I is provided which is supported by means of an insulating block 42 and has a conductor 43 connected thereto. The conductors 39 and 43 are connected to suitable terminals within the control apparatus I4, as will be shown in connection with the discussion of Fig. 2.

The drive motor I2 is provided with suitable gearing 44 for directly driving the lead screw 2 I. and is also arranged to drive the drive shaft I5 through a differential 45. For varying the speed of the drive shaft I5 with respect to the lead screw 2 I in order to adjust the pitch of the potentiometer being wound, the lerror correction motor I3 isV also arranged to drive the drive shaft I5 through gearing 46 and the differential 45.

The differential 45 is one of the in-line shaft type, having an input shaft carrying a gear 41 driven by the drive motor I2 through a pinion 48, a second input shaft carrying a gear 49 driven by the error correction motor I3 through the gearing 45, and an output shaft carrying a gear 5I, which is caused to rotate in the conventional manner of differential gearing at a speed equal to the algebraic difference in speed between the driven gears 41 and 49. For connecting the drive shaft I6 to the gear 5I, a gear 52 is provided.

For enabling suitable speed control to be provided, the motors I2 and I3 may have two tachometers of the electrical type, such as permanent-magnet generators 53 and 54, respectively, mechanically connected thereto. It is to be understood that the motors I2 and I3 and the generators 53 and 54 have electrical connections to suitable terminals in the control apparatus I4, but these connections are omitted' from Fig. l for simplicity of the drawing.

In order to enable the resistance of the potentiometer being wound on the card 21 to be checked continuously, a master potentiometer 55 is provided having a brush 55 and a suitable mechanical inter-linkage 51 to the lead screw 2l for causing the resistance of the potentiometer 55 to be varied progressively as wire is being fed onto the card 21 at a rate determined by the rate of rotation of the lead screw 2I, which also represents the rate of axial travel along the potentiometer card 21.

If desired, the master potentiometer 55 may be a functional potentiometer having the desired function which is to be reproduced by the potentiometer being wound -uponfthe card 21; and the linkage 5l may be such as to cause linear rotation of the brush 56 with respect to the Vlead screw 2 i. In this case, the brush 56 is carried by a shaft 56, which may be connected directly or through gearing such as gearing 59 to the lead screw 2|.

However, in order to increase Vthe flexibility of the apparatus and to enable the same very accurately and precisely wound master potentiometer 55 to be employed for checking resistance when winding potentionieters ofdiiierent functional relationships, I nd it convenient to provide a functional linkage and to make the poten-V tionieter 55 exactly linear in its relationship between resistance and travel of the brush 56. To this end, a suitable linkage such Vas one comprising a cam 5I and a cam `follower Eil. may berproe vided. The cam 6I, as shown, is carried by-a shaft 63 connected through the gearing 59 to the lead screw 2 i, and the cam follower 62 is-carried by an larm dit secured to the potentiometer brush carrying shaft 53. if the master potentiometer 55 is precisely a linear potentiometer, any desired functional relationship for the potentiometer wound upon the card '2l may be'obtained by a suitable contour of the cam 6i. The same arrangement may be employed when the potentiometeris to be wound linearly, since in that case, the cam lift of the cam 6I may be made proportional to the rotation of the shaft 63 by shaping the cam El as a spiral. Replaceable cams of dilerent apropriate contours are provided. for the functions wbichrnay be desired in winding potentiometers.

The control apparatus It ofV Fig. l comprises electrical control circuits `for the drive motor I2 and the error correction motor i3, designated generally by the reference numerals S5 and Gt, respectively in Fig, 2.

As illustrated in Fig. 2, the electrical circuits 65 are arranged for causing the drive motor I2 to operate at a predetermined selected speed. The circuits t@ are arranged for causing the error correction motor i3 to operate at such a speed as to increase or decrease the speed of the drive shaft It sufficiently by acting through the differential t5 to cause whatever variation in pitchis required to prevent deviations from the desired functional relationship of the potentiometerbeing wound on the card 21.

The lerror correction motor control circuits 66 include a resistance measuring bridge 61, a bridge amplifier E58, an adjustable gain amplifier 69 for varying the ampliiier output in accordance with variations in speed of the lead screw, a! phase inverter and differential amplifier 'li and a pushpull power amplifier l2 for controlling the torque and direction of rotation of the error correction motor i3.

For relating the speed of the error correction motor i3 to a manually set normal speed adjustn ment, a pitch control potentiometer limay be provided with a modulator lll acted upon jointly by the potentiometer 'i3 and the permanent inag net generator 5d and having an output connection through a iilter 15 to the phase inverter and differential amplifier l l.

The electrical circuit 6b for controlling the drive motor i2 includes elements similar to some oi" those shown in the circuit 56, namely, amanual speed control potentiometer it, a modulator ifi acted u'poniby the potentiometer 'it and the permanent-magnet generator 53, a phase inverter amplifier i8, and a pushpull connected power l 6 amplifier 1.9 controlling the torque or the drive motor I2.

i If desired, an error monitoring circuit tI may be provided. v

For suitably energizing and biasing the amplifiers, positiveandnegative, or B and 0, power supply sources may be'provided which are represented by positive terminals 82 and negative terminals 33,A respectively.

vThe .resistance-measuring Wheatstone bridge t1 includes the usuali| four arms and comprises aipair ofstandard ratio arms Btl and 85, an adjustable'standard resistance arm consisting of a resistor 86 in series with the portion of themaster potentiometer 5b between its end terminal 81 and'its brush 56, and a variable or unknown resistance arm consisting of an initially balancing rheostat 8B in series with the Wire 25 which has already been wound upon the potentiometer card 21. l'

For energizing the bridge 61, either alternating current or direct current may be employed, but in the arrangement illustrated, a 60cycle central station source is. employed, and a step-down transiorIner'Q-'is provided, having -a secondary winding 9b connected to input terminals of the bridge 67. One of the diagonal-arm conjugate points BI is grounded and the other diagonal-arm point 92 is connected to the bridge amplier 68.

The ampliiiers are of the electric discharge device type, such as triode and multigrid vace uum tubes, for example. VvThe bridge amplier B3 is shown as a Ytriode having a grid @3 connected to the ungrounded diagonal arm point 92 of the resistance-ineasuring"bridge' Si, and having an output coupled to the amplifier til.

The ampliiier 69 has an input grid 94 coupled to the amplifier S8 and biased to a potential varying with the speed of the lead screw 2! (Fig. l) for the purpose of making the gain of the amplifier E9 responsive thereto. The ungrounded brush of the lead rscrew vspeed measuring generator 53 is connected through resistors Si! to the grid 94 of the amplier 69. For smoothing out fluctuations a by-pass condenser IGF) is provided.

In order to stabilize the error correction motor I3 and prevent hunting, the pitch adjustment tachometer generator 54 may be coupled to the error motor control circuit Gt through the modulator 14 to introduce a correction proportional to motor speed.

The Vmodulator 'I4 comprises a pair of rectiiiers IDI and |02 connected in series to a winding of suitable voltage, such as `a 50 volt winding |03 of the transformer 89, the center tap IM of the winding H13 being grounded.

For producing a voltage the .magnitude of which `determines the speed of the error correotion motor I3 when the pitch error is Zero, there is provided a voltage divider comprising a resistor and a resistor Ict Vconnected in series between the vungrounded terminal of the permanent magnet generator 54 and the brush It? of the manual pitch control potentiometer i3, Vwhich a junction terminal IBB connected to the junction terminal of the modulators Iii! and m2. For anticipating errors by making the pitch adjusting speed correction proportional to accclerae tion of the errorcorrection motor I3, the resistor |65 may be shunted by a condenser E89. In order to increase the relative effect or resistance error a resistor lfIIl may be connected between the terminal |88 andV ground.`

The junction terminal M18 o f'themodulator 14 is coupled to the filter 15 through a cathode follower |09.

The phase inverter amplifier 1| comprises a tube III coupled to the variable gain amplifier 69 and a tube H2 coupled to the filter 15. The tubes HI and H2 are so connected as to give a differential output proportional to difference between the alternating-voltage output of the filter 15 and the output of the amplifier 69. The lter 15 is tuned to the frequency of the source supplying the transformer 89 in order to convert the output of the modulator 14 into a sine wave which may be opposed to the sine wave from the amplifier 69 in the phase inverter 1I.

The error corrector motor I3 may be an alternating current motor of the servo type having a field winding H3 with a center tap H4. The winding H3 is connected between plates of the tubes ||5 and H6 forming the pushpull power amplifier 12 coupled to the phase inverter ampliiier. The center tap H4 is connected to the power supply positive terminal 82.

The modulator 11 is similar to the modulator 14 but may be connected to end terminals H1 and I I8 of a conventional 6.3-volt filament-heating transformer winding (not shown) the connections of which are omitted for simplicity in the drawing, but-which has the usual grounded center tap.

The phase inverter amplifier 18 compirses a tube H9 with a grounded grid and a tube |2| with a. control grid |22 connected to the output or junction terminal |23 of the modulator 11.

l The power amplifier 19 is coupled to the phase inverter ampliiier 18 similarly to the arrangement of amplifiers 12 and 1|.

The drive motor I2 may also be an alternating current motor of the servo type having a field winding |23 differentially connected in the load circuits of the tubes constituting the amplifier 19, with a center tap |24 connected to the positive power supply terminal 82.

In the arrangement illustrated the lead-screwspeed-responsive tachometer generator 53 is also provided with a voltage divider, consisting of resistors 95 and 96 connected in series between the brush 91 of the manual speed-control potentiometer 16 and the ungrounded terminal of the generator 53. The resistors 95 and 96 have a junction terminal 98.

As represented schematically by the broken line |25, the error motor I3 is mechanically connected to the card 21 with the differential 45 interposed in this connection. The error motor I3 is also directly connected to the permanent magnet generator 54 but the mechanical connection is omitted in Fig. 2 for the sake of simplicity in the drawing.

As represented by the broken line |26, the drive motor I2 is directly connected to the permanent magnet generator 53 and is also connected by means (not shown in Fig. 2) to the mechanism for advancing the wire feeding carriage 28.

The potentiometer brush 56, it will be understood, is also driven by the drive motor I2 in accordance with the desired function represented by the linkage 51 in Fig. 1, omitted for the sake of simplicity from Fig. 2.

If desired for the protection of the machine a limit stop safety switch I4I may be provided which is supported by or suitably connected to the lead screw carriage 28 and carries `a switch operating arm |42 adapted to engage a stop such as the tip |43 of a screw |44. As shown in Fig.

2, 'the switch |4| comprises a pair or pairs of normally-closed, relatively movable contacts |45 and |46 connected in suitable portion of the circuit such as in series with the main power supply to the drive motor I2 and the error motor I3.

When the machine is set up for automatically winding potentiometer wire on the card 21, the manual speed control potentiometer 16 is set at the position corresponding to the desired speed at which the lead screw 2| is to be driven, which also determines the normal speed of the drive shaft I6 assuming a normal diameter and resistivity of the wire 26. The gears 44 are chosen to give a speed ratio producing approximately the correct pitch if the Wire 26 has the expected resistance per unit length. Assuming that the ungrounded brush of the generator 53 is at positive potential when the drive motor I2 is rotating forward, the speed adjusting brush 91 is set at a negative voltage substantially equalling (actually slightly exceeding) the voltage of the generator 53 at the desired speed. If the drive motor I2 should operate at a speed less than that correspending to the position at which the brush 91 of the potentiometer 16 has been set, then the voltage produced by the permanent magnet generator 53 would be less than that supplied by the brush 91. The junction terminal 98 will fall so far below ground as to increase the square wave output of the modulator 11 accordingly. The modulator tubes are rendered conducting on alternate cycles of the alternating-current supply so as to connect the grid |22 to the grounded transformer mid-tap. Otherwise, the grid |22 is tied to the junction terminal 98. In this manner a square wave with a peak Voltage dependent on the voltage of the terminal 98 is applied to the grid of tube I2 I. This increases the voltage swing of the grid |22 of the phase inverter amplifier 18, and increases the output of the amplifier 19 so as to increase the torque of the motor I2, and raise its speed until the potential of the junction terminal 98 has risen to the value at which the modulator output is just sufcient to produce adequate torque for driving the motor I2. This necessitates that the point 98 always remain slightly negative for the polarities of connections assumed.

In the event that the resistance measuring bridge 61 should be unbalanced by a deviation between the resistance of the potentiometer 55 and the wire 26, an alternating voltage will be supplied to the grid 93 of the bridge amplifier 68 and will be transmitted through the amplifier 69 to the grid I I of the phase inverter amplifier 1|. The gain of the amplifier however, will be adjusted according to the speed of the drive motor I2 .and the generator 53, which determines the lead screw speed in order that correction will take place more rapidly when the apparatus is set for high pitch in the winding 26. The magnitude of the voltage supplied to the grid 93 depends on the magnitude of the error; and its phase (0 or 180) depends on whether the lead screw speed is too fast or too slow.

Assuming first that the manual pitch control brush |01 is set at zero or ground voltage so that the manual pitch control is ineffective, the grid of the tube I I2 will remain at ground potential. If there is no error in pitch, the voltage supplied to the grid of the tube I I I is zero and the latter grid also remains at ground potential being so biased by its grid leak. No voltage is supplied to the winding H3 of the error correction motor I3 and it remains stationary. Referring l 'it to rig. v1, the gear s ofthe differential' 45 re- 'main's stationary, 4and the gear 5I rotates at the speed'of the gear 41. This is equivalent 'to a direct drive between the drive motor I2 andthe card drive shaft I6. In case any error in resistance develops, however, a voltage appears at bridge 6l (Fig 2) andthe error correction motor I3 rotates forward or backward according to the phase of the error voltage. Acting through `the differential d5, the motor |3 thus increases or decreases the speed of the drive shaft I6 suciently to correct the pitch and make the resistance of the potentiometer being wound conform to that of the master v55.

In order to avoid the necessity for many sets of gearsffll (Fig. 1) with different ratios forwi'nd- Ving potentiometers with different pitches, the er- The speed of the lead screw 2| comuputed as Y proper assuming expected resistivity and diameter of the wire 26, is set by means of the pitchcontrol potentiometer '13. The brush |01 is set at a voltage above or below ground depending in the desired direction of rotation of the motor I3. In case the error control motor I3 deviates from the speed corresponding to the pitch adjustment i of the pitch control potentiometer 13, the permanent magnet generator 54 produces an output voltage above or below that of the brush |81 of the potentiometer 13, which determines the phase of the output of the modulator H, thus causing the motor |3 to run in one direction or the other since its armature is supplied with current of fixed phase. The amplitude of the voltage also depends upon the magnitude of the deviation from preset speed. If the correct setting of the pitch adjustment I3 has been accurately estimated and the wire remains uniform, the output of the bridge El is Zero and the entire voltage fork controlling the motor I3 is supplied through the modulator M and the tube -I I2. In the event of error, however, voltage appears at the bridge E1 and is supplied to the tube I I I.

The phase inverter amplier tube III acts in opposition to the tube I l2 (or aiding according to the direction of error) and the net output is supplied to the amplifier 'I2 for increasing or decreasing the speed of the error correction motor I3.

The pitch motor speed has three components respectively proportional to (l) the manual pitch control voltage set by the brush |01 (2) the error in resistance 3) a rate voltage including an acceleration voltage. In other words the tachometer generator 54 must run with such speed and acceleration that the algebraic difference between the manual pitch control voltage and a voltage proportional to resistance error, balances voltages proportional to the speed and the acceleration of the generator 54. The proportion of the acceleration effect may be'varied by varying the size of the condenser |09 and the proportion of the speed effect may be varied by varying the voltage division ratio of the resistors |05 and I Ill. The proportional effect of the manual pitch control voltage may be varied by varying autres 1Q the resistance of' the 'resistor H15.' The resist'- ance error may be considered as an integrated error in pitch. Y

If it is desired to observe the magnitude and` direction of the error which represents the deviation from computed resistance oithe wire 2S, the error monitor 8| may be employed, which comprises an input amplifier I2? coupled tothe outputof the variable gain amplifier Sie and having an output transformer |23 supplying a phase sensitive circuit consisting of a reference voltage source |129 and rectiers IBI `and |32 with condenser-shunted' load-'resistors |33 and |34, having ajunction-terminal |35.' The reference voltage source |29 islconnected between the junction terminal |35 and the mid-terminal |3il of the transformer 'i2-B. -A suitable direct-currentresponsive instrument suchas a center-zero volt meter or galvanom'eter |355 is connected across resistors |33 and |34 to indicate the polarity of the voltage across the resistors |33 and |34 which, in turn, represents the phase relationship between the voltage output of the amplier 69 and the reference voltage source |29. The

monitor 8| may be used to enable an operator to control the pitch manually if desired.

I have herein shown and particularly described certain embodiments of my invention and certain methods of operation embraced therein for the purpose of explaining its principle of operation and showing its application, but it will be obvious to those skilled in the art that many modifications and variations are possible, and I am, therefore, to cover all such modifications and variations as fall within the scope of my invention which is dened in the appended claims.

What is claimed is:

l. In a potentiometer Winder having winding form rotating means and lead screw driving means, with means for varying the speed ratio between said winding form rotating means and said lead screw driving means to produce a predetermined resistance function of wire wound on the winding form, an adjustable gain ampliiier for driving said ratio adjusting means, a generator coupled to said lead screw and adapted to produce an output that depends on the speed of said lead screw, and means for applying said output to said amplifier as a bias for varying the gain of said amplifier.

2. A motor speed control system comprising in combination with a motor, the speed of which is to be controlled, a tachometer generator driven thereby, a voltage source serving as a speed standard connected to said tachometer generator with a junction terminal, the potential of which is responsive to unbalance between the voltage source and the tachometer generator output, a motor control circuit connected to said motor and a thermionic modulator interposed between said junction terminal and said control circuit, thus supplying an output voltage, the magnitude of which is proportional to deviation between said voltage source and the output of said tachometer generator for varying the torque of the motor to adjust its speed in order to approach a balance between said tachometergenerator and said voltage source.

3. In an automatic motor control circuit, an error responsive voltage source for Supplying a voltage proportional to error for correcting motor speed, a phase inverter thermionic amplier and a manually adjustable correcting voltage source, said phase inverter amplifier having input connections from said automatic error volt.-

Vage source, and Van output connection to the mopotentiometer and a rotatable Winding form driven simultaneously with means for making electrical connections to the master potentiome- -ter and the portion of the wire wound on the wire form, a resistance comparator connected to said electrical connections, a winding pitch control motor and an amplifier interposed between said resistance comparator and said motor for varying the operation of the motor in response to deviation between resistances of the master potentiometer and the portion ofthe Wire wound upon the winding form, an anti-hunt generator mechanically connected to the pitchcontrol motor and electrically coupled to the motor controlling amplifier, and a manual pitch adjustment comprising an adjustable voltage source electrically connected to balance out an adjustable voltage from the anti-hunt generator to supply a predetermined error correction voltage to the amplifier and thereby relieve the resistance comparator.

HERBERT HARRIS, JR.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,048,548 Kramer Dec. 31, 1912 1,057,008 Phillips Mar. 25, 1913 1,699,024 Schnitzer Jan. 15, 1929 2,319,413 Leathers et al May 18, 1943 

